Particles of hexagonal boron nitride (hereinafter referred to as “hBN”) have a layer structure similar to that of graphite and have excellent characteristics such as high heat conductivity, electrically insulating properties, chemical stability, solid lubricity, and thermal shock resistance.
By taking advantages of these characteristics, an electronic materials field employs a heat dissipative member incorporated with the hBN powder in a resin or rubber, such as a heat dissipative grease, a flexible spacer, and a heat dissipative sheet, in order to efficiently disperse heat generated from an electronic component.
A common hBN powder is an aggregate of scaly particles and when the hBN powder is incorporated in a resin or rubber, the particles are aligned in the same direction to be oriented (see JP-A No. 9-202663). The hBN particles have a thermal conductivity of 400 W/mK in a surface direction (a-axis direction) but have a thermal conductivity of 2 W/mK in a thickness direction (c-axis direction), which suggests anisotropic thermal conductivity. Thus, for example, when the hBN particles are oriented in a heat dissipative sheet, the hBN particles are incorporated while a surface direction (a-axis direction) of the particles is in parallel with a surface direction of the sheet, and hence the sheet is unlikely to obtain an improved thermal conductivity in the thickness direction.
In order to solve such a problem, an hBN powder that is unlikely to be oriented even when it is incorporated in an insulating heat dissipative sheet and that has a shape except the scaly shape has been intended to be used. Examples of such an hBN powder include a bulk of boron nitride containing an undeveloped crystal (see JP-A No. 61-117107), an hBN powder granulated by spray drying or the like (see U.S. Pat. No. 6,348,179), and an hBN powder produced by pulverization of a sintered hBN (see JP-A No. 9-202663).
However, the bulk of hBN containing an undeveloped crystal is inferior in characteristics such as purity and heat conductivity to the scaly hBN powder, and hence the insulating heat dissipative sheet does not have improved thermal conductivity in the thickness direction and also has reduced moisture resistance reliability. The hBN powder granulated by spray drying or the like has a particle size of about 10 μm or less and a spheroidized boron nitride having a large particle size is difficult to be synthesized. The powder produced by pulverization of a sintered hBN has some advantages compared to conventional powders because the hBN particles are oriented during hot press or preformimg in a production process of the sintered hBN to increase the ratio of particles in which the oriented primary particles are aggregated. However, the hBN particles are still incorporated while the a-axis direction is in parallel with the surface direction of the insulating heat dissipative sheet. Moreover, the powder requires high cost.
Meanwhile, for a reduction nitridation method for synthesizing hBN from an oxide of boron and carbon as raw materials under a nitrogen atmosphere, there are disclosed a method for synthesizing scaly boron nitride (see JP-A No. 60-155507), a method for synthesizing boron nitride nanotube (see JP-A No. 2000-109306), and a method for synthesizing hBN having a large surface area (see Non-Patent Document: Nano Letters, 2004, 4 (1), 173-176). However, there is no description about a synthesis of spheroidized boron nitride.